The distribution of Fe in the Australian sector of the Southern Ocean

The vertical profiles of labile Fe in seawater in the Australian sector of the Southern Ocean (50–65°S along 140°E) were obtained in the austral spring 1994/1995 by concentration with 8-hydroxyquinoline immobilized fluoride containing metal alkoxide glass (MAF-8HQ) followed by determination with chemiluminescence on board. While the concentrations of Fe were low in the surface water column (0.14±0.12 nM, n=97 for 0–100 m) and increased in deep water at all stations, the profiles varied depending on the latitude. The Fe concentrations in intermediate and deep water were 0.30±0.12 (n=36 for 500–2000 m) between the Antarctic Front (AF) and Antarctic Divergence (AD), and steeply increased south of the AD. The high Fe region (<1.5 nM) extended northwards along the continental slope and coincided with the Antarctic Bottom Water. Another Fe maximum in deep water (∼1.2 nM) was located on the north side of the Pacific Antarctic Ridge. The Fe concentrations showed a minimum (∼0.3 nM) at 3000 m depth at 62°S, which was almost coincident with the maximum of SiO2 (∼125 μM) in the Lower Circumpolar Deep Water. There were some patchy Fe maximums in shallow water between the AF and AD, which did not coincide with the maximums of PO4 and NO3 in the Upper Circumpolar Deep Water. These results indicate that the distribution of Fe is nutrient-like but strongly influenced by local sources and water circulation. : PO4 ratios in the surface water column (<3×10−4) were lower than the critical value for a phytoplankton bloom (1×10−3) proposed by de Baar et al. (1990, Marine Ecology Progress Series 65, 105–122). Although the concentrations of Fe did not correlate with those of chlorophyll a, they positively correlated with the indices of primary productivity (growth rate, productivity by simulated in situ 13C method and monthly integrated net community production). It seems that phytoplankton suffered Fe stress and that Fe was a limiting factor of the primary production. However, it is likely that there were some co-limiting factors, such as grazing at the ice edge and depletion of SiO2 and critical-depth/mixed-depth relationship in the Permanently Open Ocean Zone and Polar Front Zone.